1. Technical Field
This invention relates generally to an exhaust gas aftertreatment system for internal combustion engines and more particularly to a NOX aftertreatment system for lean burn engines.
2. Background Art
Worldwide emissions regulations slated for introduction during the next five to ten years will require that gasoline and Diesel engines be equipped with some form of exhaust aftertreatment device. Perhaps of most concern to the Diesel engine industry are the proposed reductions in NOX (the various oxides of nitrogen) emissions, because they are the most difficult to eliminate from the exhaust stream of fuel-lean, better known as lean burn, combustion. Lean burn gasoline and Diesel engines offer the benefits of higher thermal efficiency, but suffer from difficulty with NOX emissions. Nitrogen is present in the air we breathe, and in the air that an engine consumes. Nitrogen replaces the air by approximately 75%. Nitrogen does not burn, but it can oxidize at temperatures over 2500xc2x0 F. NOX ix a health hazard and one of the EPA""s primary emission problems.
Without a major advance in catalyst technology, automobiles using lean burn engines will not be able to meet current and projected emission standards. Two of the main candidate technologies being considered for use in Diesel engines are selective catalytic reduction (SCR) using urea or other means to generate ammonia, and lean NOX adsorber catalysts. Neither one of these technologies has yet demonstrated the ability to meet the very stringent U.S. 2007 standards. Moreover, both technologies have considerable drawbacks. Selective catalytic reduction currently requires that a urea/water mixture be carried on-board and injected into the exhaust system, or some other form of on-board bulk ammonia generation. If excessive urea is injected, excessive ammonia (NH3) is generated and there is a possibility of emitting toxic ammonia into the atmosphere. Lean NOX adsorber catalysts must be regenerated periodically by passing products of rich combustion through the adsorber. In order to obtain the highest NOX conversion efficiency possible, more rich products must be passed through the adsorber than are required to reduce the trapped NOX. However, this has recently been discovered to produce NH3 in the exhaust stream. As noted above, NH3 is toxic and must be prevented from being discharged into the atmosphere. The only alternative heretofore has been to pass the gas containing NH3 through an oxidation catalyst where it forms NO in the presence of an oxidizing atmosphere. However, this is not desirable since the aim of the aftertreatment itself is to eliminate NO and NO2 in the first place.
An alternative for regenerating NOX adsorption catalysts is proposed in U.S. Pat. No. 6,176,079 B1 issued Jan. 23, 2001, to Konrad, et al. for a PROCESS AND APPARATUS FOR REDUCING NITROGEN-OXIDE EMISSIONS IN EXHAUST GAS, proposes using three serially connected catalyst units; a NOX adsorption catalyst followed by separate ammonia-producing and ammonia-adsorption catalysts. The process proposed by Konrad, et al. is directed at maximizing the production of ammonia. In similar fashion, U.S. Pat. No. 6,119,452 issued Sep. 19, 2000 to Kinugasa, et al., and more recently, U.S. Pat. No. 6,338,244 B1 issued Jan. 15, 2002, to Guenther, et al. are also directed to the intentional production of ammonia to optimize nitrogen-oxide emissions reduction. However, as noted above, ammonia is a toxic material and its release into the atmosphere must be prevented.
The present invention is directed to overcoming the problems associated with the intentional production of ammonia as proposed by Konrad, et al., Kinugasa, et al., and Guenther, et al. It is desirable to have a high efficiency NOX aftertreatment system that does not require the intentional production of large amounts of ammonia which could be released into the atmosphere if the ammonia-adsorption catalyst becomes saturated. It is also desirable to have a high efficiency NOX aftertreatment system for use with lean burn engines that advantageously uses ammonia undesirably generated when the engine control system is optimized to maximize the NOX conversion efficiency of the NOX adsorber system. This important feature of the present invention allows engine control systems to be tuned for the very high NOX conversion efficiency demanded by future engine emissions laws.
In one aspect of the present invention, a NOX aftertreatment system for a lean burn internal combustion engine adapted to selectively operate normally in a lean burn combustion mode and periodically in a defined fuel-rich combustion mode includes an exhaust system in fluid communication with at least one combustion chamber, a NOX adsorber and a selective reduction catalyst. The NOX adsorber is disposed in the exhaust system and is adapted to store at least a portion of NOX produced during operation of the engine in the lean burn combustion mode. The NOX stored in the NOX adsorber is reduced, and the NOX adsorber regenerated, during operation of the engine in the defined fuel-rich combustion mode. The defined fuel-rich combustion mode includes continued operation of the engine in a fuel-rich combustion mode for a period of time sufficient to provide more products of combustion than required to reduce the NOX stored in the NOX adsorber, whereby a portion of the excess products of combustion form ammonia. The selective reduction catalyst is also disposed in the exhaust system downstream of the NOX adsorber and is adapted to store ammonia formed during periodic operation of the engine in the defined rich combustion mode. During normal operation of the engine in the lean burn combustion mode, the stored ammonia provides a reducing agent for NOX remaining in the exhaust gases after passage through the NOX adsorber during normal operation of the engine in the lean burn combustion mode.
Other features of the NOX aftertreatment system embodying the present invention include a particulate matter filter disposed in the exhaust system of the NOX aftertreatment system.
In another aspect of the present invention, a method for reducing NOX emissions from a lean burn internal combustion engine includes normally operating the engine in a lean combustion mode and storing NOX generated during operation of the engine in the lean combustion mode in a NOX adsorber positioned in an exhaust system of the engine. The method further includes periodically operating the engine in a defined rich combustion mode in which the defined rich combustion mode includes continued operation of the engine in the fuel-rich operating mode for a period of time sufficient to provide more products of combustion than required to reduce the NOX stored in the NOX adsorber during operation of the engine in the lean combustion mode. During the periodic operation of the engine in the defined fuel-rich combustion mode, the portion of the excess products of combustion form ammonia. The ammonia formed during the defined fuel-rich operating mode is stored in a selective reduction catalyst disposed in the exhaust system downstream of the NOX adsorber and, during operation of the engine in the normal lean combustion mode, NOX remaining in the exhaust gases after passage of exhaust gases through the NOX adsorber are reduced by the selective reduction catalyst.
Other features of the method for reduced NOX emissions from a lean burn internal combustion engine include passing exhaust gases discharged from a combustion chamber of the engine through a particulate matter filter prior to discharge of exhaust gases into the atmosphere.